POLARISATION OF LIGHT. 137 



where it is first observed. In the accompanying diagram, 

 fig. 81, the line appears double, as a b and c d, or the dot, 



as e and /. Or allow a ray of light, g h y to fall thus on the 

 crystal, it will in its passage through be separated into two 

 rays, hf, he; and on coming to the opposite surface of the 

 crystal, they will pass out at ef in the direction of i k, 

 parallel to g h. The plane I m n o is designated the prin- 

 cipal section of the crystal, and the line drawn from the 

 solid angle I to the angle o is where the axis of the crystal 

 is contained; it is also the optic axis of the mineral. Now 

 when a ray of light passes along this axis, it is undivided, 

 and there is only one image; but in all other directions 

 there are two. 



If two crystals of Iceland spar be used, the only differ- 

 ence will be, that the objects seem farther apart, from the 

 increased thickness. But if two crystals be placed with 

 their principal sections at right angles to each other, the 

 ordinary ray refracted in the first will be the extraordinary 

 m the second, and so on vice versd. At the intermediate 

 position of the two crystals there is a subdivision of each 

 ray, and therefore four images are seen ; when the crystals 

 are at an angle of 45 to each other, then the images are 

 all seen of equal intensity. 



Mr. Nicol first succeeded in making rhombs of Iceland 

 Bpar into single-image prisms, by dividing one into two 

 equal portions. His mode of proceeding is thus described 

 in the Edinburgh Philosophical Journal (vol. vi. p. 83) : 



